Phased array antennas can create and electronically steer a beam of radio waves in varying directions without physical movement of the radiating elements therein. As shown by FIG. 1A, in a phased array antenna 10, radio frequency (RF) feed current is provided from a transmitter (TX) to a plurality of spaced-apart antenna radiating elements via phase shifters (ϕ1-ϕ8), which establish a desired phase relationship between the radio waves emitted by the spaced-apart radiating elements. As will be understood by those skilled in the art, a properly established phase relationship enables the radio waves emitted from the radiating elements to combine to thereby increase radiation in a desired direction (shown as θ), yet suppress radiation in an undesired direction(s). The phase shifters (ϕn) are typically controlled by a computer control system (CONTROL), which can alter the phases of the emitted radio waves and thereby electronically steer the combined waves in varying directions. This electronic steering can be important when the phased array antennas are used in cellular communication and other RF-based systems.
For example, in a typical cellular communications system, a geographic area is often divided into a series of regions that are commonly referred to as “cells”, which are served by respective base stations. Each base station may include one or more base station antennas (BSAs) that are configured to provide two-way radio frequency (“RF”) communications with mobile subscribers that are within the cell served by the base station. In many cases, each base station is divided into “sectors.” In perhaps the most common configuration, a hexagonally shaped cell is divided into three 120° sectors, and each sector is served by one or more base station antennas, which can have an azimuth Half Power Beam Width (HPBW) of approximately 65° per sector. Typically, the base station antennas are mounted on a tower or other raised structure and the radiation patterns (a/k/a “antenna beams”) are directed outwardly therefrom. Base station antennas are often implemented as linear or planar phased arrays of radiating elements. For example, as shown by FIG. 1B, a base station antenna 10′ may include side-by-side columns of radiating elements (RE11-RE18, RE21-RE28), which define a pair of relatively closely spaced antennas A1 and A2. In this base station antenna 10′, each column of radiating elements may be responsive to respective phase-shifted feed signals, which are derived from corresponding RF feed signals (FEED1, FEED2) and transmitters (TX1, TX2) and varied in response to computer control (CONTROL1, CONTROL2).
Unfortunately, these phase-shifted feed signals are typically provided across multiple mechanical components before reaching a corresponding radiating element and these signal paths may degrade the feed signals by introducing unacceptable levels of passive intermodulation (PIM) distortion. As will be understood by those skilled in the art, PIM is the generation of interfering signals caused by nonlinearities in one or more mechanical components of a wireless system. Typically, two signals will mix together (amplitude modulation) to produce sum and difference signals (and products within the same band) and thereby cause interference. PIM is a problem in almost any wireless system, but is most noticeable in cellular base station antennas, transmission lines, and related components.
Junctions of dissimilar materials (e.g., different metals) are a prime cause of PIM. Thus, PIM may occur in antenna elements, coaxial connectors, coaxial cable, and grounds. PIM can also be caused by rust, corrosion, loose connections, dirt, oxidation, etc. Even the presence of nearby metal objects, such as guy wires, anchors and roof flashings may cause appreciable PIM by creating diode-like nonlinearities that operate as mixers. And, as the degree of nonlinearity increases, so do the amplitudes of the PIM signals. PIM may also increase as components age and in environments where there are wide temperature variations, salt air or polluted air, or excessive vibrations.
Referring now to FIG. 2, conventional methods 20 of manufacturing feed circuit boards for base station antennas BSAs, which may be susceptible to relatively high levels of PIM distortion, may include forming a printed circuit board (PCB) having a through-hole (TH) therein, which may be an unplated through-hole (UTH) or a plated through-hole (PTH), and a patterned metal trace on a front side of the PCB, Block 22. As shown by Block 24, a coaxial feed cable having a shielded (and grounded) outer conductor and a signal-carrying inner “feed” conductor is mounted to a back side of the PCB, with the inner conductor extending through the through-hole TH and the outer conductor being soldered to a ground plane on the back side of the PCB. Next, as shown by Block 26, the front side of the PCB is soldered to thereby electrically connect the inner conductor within the through-hole TH to a metal trace on the front side of the PCB. Finally, as shown by Block 28, the PCB and metal trace thereon are electrically coupled (directly or indirectly) to a radiating element within an antenna assembly, using conventional operations known to those skilled in the art.